In recent years, a variety of power storage devices, for example, secondary batteries such as lithium-ion secondary batteries, lithium-ion capacitors, and air cells have been actively developed. In particular, demand for lithium-ion secondary batteries with high output and high energy density has rapidly grown with the development of the semiconductor industry, for electronic devices, for example, portable information terminals such as mobile phones, smartphones, and laptop computers, portable music players, and digital cameras; medical equipment; next-generation clean energy vehicles such as hybrid electric vehicles (HEVs), electric vehicles (EVs), and plug-in hybrid electric vehicles (PHEVs); and the like. The lithium-ion secondary batteries are essential as rechargeable energy supply sources for today's information society.
For the power storage devices, high capacity, high performance, safety in various operating environments, and the like are required.
To satisfy the above requirements, electrolytic solutions for power storage devices are under active development. Cyclic carbonates are used as organic solvents in electrolytic solutions for power storage devices. In particular, ethylene carbonate is often used because of its high dielectric constant and high ionic conductivity.
However, not only ethylene carbonate but also many other organic solvents have volatility and flammability. For this reason, in the case of using an organic solvent for an electrolytic solution for a power storage device, the temperature inside the power storage device might rise due to a short circuit, overcharge, or the like and the power storage device might burst or catch fire.
In view of the above, the use of an ionic liquid (also referred to as a room temperature molten salt) having non-flammability and non-volatility as a nonaqueous solvent for a nonaqueous electrolyte of a lithium-ion secondary battery has been proposed. Examples of such an ionic liquid include an ionic liquid containing an ethylmethylimidazolium (EMI) cation, an ionic liquid containing an N-methyl-N-propylpyrrolidinium (P13) cation, and an ionic liquid containing an N-methyl-N-propylpiperidinium (PP13) cation (see Patent Document 1).
Improvements have been made to an anion component and a cation component of an ionic liquid to provide a lithium-ion secondary battery that uses the ionic liquid with low viscosity, a low melting point, and high conductivity (see Patent Document 2).